When drilling an oil well or the like, a steel casing is commonly set in concrete near the ground surface for containing drilling mud and production fluids, and maintaining well pressure as the well is completed. The well is drilled by rotating a drill bit on the end of a steel pipe or drill string that extends through the casing to the bottom of the hole being drilled. Drilling mud is pumped into the drill string at high pressure during drilling for a variety of purposes, including cooling the drill bit, carrying particles of the formation being drilled to the ground surface, and providing hydraulic head in the casing for controlling well pressure. The drilling mud circulated down the drill string returns in the annulus between the casing and drill string.
The drilling mud is discharged from the casing through an adjustable drilling choke or valve. The choke limits the discharge of mud, thereby maintaining a desired back pressure in the casing for well control. Such a drilling choke may, for example, be a high-pressure needle valve having a relatively small orifice through which the mud flows.
Sometimes chips of formation being drilled are too large to pass through the adjustable choke when it is partly closed for maintaining a set point pressure in the well. Such particles can partly plug the choke, causing pressure in the casing to rise. It is undesirable for the casing pressure to exceed a selected limit for a variety of reasons, such as, for example, blow-outs, damage to the well head, casing or related equipment, excessive back pressure on the mud pumps, and damage to the well formation itself.
Thus when the casing pressure rises, indicative of plugging the choke, it is desirable to rapidly open the choke to its fully open position, thereby relieving pressure and ordinarily clearing the choke of the obstructing debris. Since rapid response is required, an automatic rather than manual system is desirable. It is also important that after the casing pressure has dropped to a safe level that the drilling choke be closed to its original set point for continued drilling. Again, it is desirable that this be done automatically.
Cameron Iron Works of Houston, Tex., has provided a system for limiting the maximum allowable casing pressure during drilling. This is described in a document entitled "Drilling Choke Control System, 38933 Series Console" dated July 1982 at pages 2-17 through 2-29 and 5-27 to 5-28. The Cameron drilling choke control system employs pneumatic pressure for operating the choke actuator, using roughly 100 psig. air to drive the actuator. The high pressure in the choke manifold is converted to a low pressure analog pneumatic signal proportional to manifold pressure. The pneumatic signal is compared with a pneumatic set point for detecting an allowable maximum pressure for opening the choke if the set point pressure is exceeded. The pneumatic pressure used to operate the choke is also employed in the Cameron system to maintain memory of the original set point of the choke for restoring the choke to its original position when manifold pressure again drops below the maximum allowable pressure.
Such a pneumatic signal has several shortcomings. It is relatively insensitive to pressure changes so that the maximum allowable pressure is not closely controlled. Quite often it is necessary to place the control system a substantial distance from the choke, and under these circumstances, there are pressure and time lags inherent in the system that can delay opening of the choke. There must be extensive recalibration of the system on a regular basis to maintain any semblance of control accuracy.
It is often desirable to employ a choke control system using hydraulic pressure at, for example, 1,500 psig. for actuating the choke. In addition to the superior reliability of a hydraulic system to a pneumatic system, the hydraulic system can provide substantially higher choke actuator torque and can employ considerably smaller components at the normally crowded site of the actuator. Further, in practice of this invention, there is provided direct sensing of the choke manifold pressure for switching a "digital" pneumatic pressure when a maximum allowable casing pressure is exceeded. By switching pneumatic pressure on or off in direct response to pressure in the drilling choke manifold, a significant source of error and time lag, the analog pneumatic signal, is eliminated. The switched pneumatic pressure not only controls hydraulic operation of the choke actuator, but also assures retention of the original actuator position set point for resetting the choke when casing pressure drops below the allowable maximum. By using the lower pressure pneumatic signal instead of the higher hydraulic actuator pressure, lower cost components can be used in the control system.